1. Field of the Invention
The present invention relates to technology of improving voice signal in quality suitable for a radio environment of a signal transmission path in an Adaptive Differential Pulse Code Modulation (ADPCM) coding system utilized for voice communication such as digital mobile communication.
2. Description of the Related Art
In voice signal coding processing with the ADPCM system provided for in ITU-T Recommendation G. 7 26, a current input signal is predicted from a past input signal, and the difference between the predicted value and the actual value is quantized and transmitted as an ADPCM code. When the ADPCM code is erroneously transmitted, in a voice communication using the ADPCM coding system, a click noise (allophone) is occurred upon converting the ADPCM code into a PCM code with an ADPCM decoder. Here, a device for compensating such transmission error of a code will be required when such voice signal coding processing is used for digital mobile communication, etc.
Japanese Patent Laid-Open Publication Hei No. 8-37504, for example, proposes a device for improving voice signal in quality as shown in FIG. 4. In FIG. 4, an error detector 12 inspects whether received ADPCM code contains any error using a Cyclic Redundancy Check (CRC), and upon an error being detected, any “0111” or “1000,” which is a code presenting the maximum difference of the ADPCM code, is replaced in a code replacement processing circuit 10 to “1111,” which is a code representing no difference. A gate signal generating circuit 13 is driven by an error detecting signal supplied from the error detector 12 and outputs a gate signal immediately after the frame (packet) in which such error detecting signal is generated.
Incidentally, when an error-free ADPCM code is received in the code replacing processing circuit 10, approximately 20 msec to 30 msec, which corresponds to 3 to 5 frames, is generally required to restore the normal internal status of an ADPCM decoder 11. Thus, the gate signal generating circuit 13 outputs a gate signal in a length corresponding to approximately 3 to 5 frames.
A click noise detector 14 determines an absolute value of an amplitude difference of adjacent PCM codes which are decoding outputs from the ADPCM decoder 11. And when the determined value exceeds a threshold value, the click noise detector 14 judges a click noise to be occurred and outputs a click noise detecting signal. In an AND circuit 15, an AND of the click noise detecting signal and the gate signal is obtained and a build-up thereof is retained in a retaining circuit 16. Pursuant to a suppressing signal from the retaining circuit 16, an ADPCM decoding output is suppressed in a suppression processing circuit 17 inserted in the input side of a PCM decoder 18. The retaining circuit 16 is reset by a reset signal representing the termination of the then-current frame. Accordingly, the suppressing signal will continue from the detection of the click noise until the termination of the then-current frame.
In the above device for improving voice signal in quality of the related art, however, radio environment of a signal transmission path is in no way taken into consideration in the click noise detector 14. For example, a click noise is not detected upon making a distinction between a click noise under the fading environment, which largely influences the voice signal grade, and the one under the Additive White Gaussian Noise (AGW) environment, which hardly influences the voice signal grade.